There are a handful of writings that no cryptologist in the world, no matter how hard they try, has deciphered. Some are intentionally encrypted, while others are probably just written in a language or alphabet that the world has forgotten. Most of these are just trivia, like the famous Kryptos in front of the CIA. A few, though, almost surely contain critically important historical information. These are the most significant, in order of age:

1.  The Indus Valley script

File:IndusValleySeals.JPGIndus Valley inscription tablets

The Indus Valley in Asia was home to a major civilization about as old and as large as Egypt, but the world forgot all about it until ruins were discovered in the 1920s. In those ruins were found tablets with inscriptions that no one has yet been able to decipher. If we could, the tablets would be a key to the history of this lost civilization, potentially as important as the Rosetta Stone.

2.  Linear A

Linear A cup.png

Linear A inscription from an Ancient Cup

Tablets inscribed in Linear A and Linear B, two similar but distinct ancient writing systems, have been found in Minoan and Mycenaean ruins on the island of Crete, which lies in the Mediterranean Sea between Greece and Egypt. Linear B has been deciphered and identified as a form of ancient Greek, but when archeologists try plugging in words or sounds from Linear B into the Linear A texts, they get bizarre gibberish. Linear A may actually encode a long-lost language totally unrelated to Greek or any language spoken today.

3.  The Voynich Manuscript

Page from the "balneological" or bathing section of the Voynich.

It doesn't get any more bizarre than the Voynich Manuscript. Written in an unidentified language in an unknown script, full of bizarre illustrations ranging from star charts to naked ladies in tubes, the book was found  in a reject pile in Italy in 1912 by Wilfrid Voynich, a rare book dealer. Carbon dating places the creation of the paper in the early 1400s. NSA cryptanalysts have tried for years and failed to decode a single word. Various (mostly ludicrous) theories have been put forward to account for the bizarre book, but a fairly plausible one surfaced last year: namely, that it was produced in Mexico in the 1500s, and records a lost language related to Nahuatl.

4.  The "Taman Shud" or Somerton Code

The Cipher Text found on the back page of Somerton Man's copy of the Rubaiyat

Okay, I said that it doesn't get any more bizarre than the Voynich, but this case is close. In 1948, an unidentified man was found dead, probably from poisoning, on a beach in Somerton, Australia. He had no identification, but he was carrying a line of poetry cut out of a book, the Rubaiyat by Omar Khayyam. A man in Somerton found the book from which the line had been cut—and written in the back was an apparent cipher message or key that no one has been able to decode. Why does this case matter? Somerton Beach is right near Woomera, the site of Australia's nuclear research facility, and the unidentified man may have been involved in espionage. The original copy of the book in which the code was found disappeared from the police file sometime in the 1950s. Suspicious yet?